they are constructed to be quite
efficient in current draw and
operate very quietly — perfect for a
large robot. Quite a few have been
used in combat robots but they
don’t exactly have the brute force
of the powerful Magmotors. NPC
also offers a series of wheels to fit
various motors.

FIGURE 7. S28-400 Magmotor.

Brushless DC Motors

Brushless motors are another
variety that have found a lot of uses in
all sizes of robots. They differ from
the brushed types mentioned earlier in
that they have a wound stator (field)
and a series of permanent magnets
on the rotor (armature) — exactly
opposite of the brushed varieties.
Since something has to produce the
rotating magnetic field, the stator
windings are electronically commutated
instead of brushes wiping a spinning
commutator. Internal circuitry
produces a three phase trapezoidal
wave form (though some motors have
a four phase circuitry). Muffin style
fans are a popular application and are
usually two phase. Since there are no
friction-producing brushes, they are
more efficient and can operate at
higher speeds. Figure 8 (from the UC
Berkeley Engineering Department)
shows a typical larger brushless motor.
Notice the electronics board that
produces the desired waveforms.

constant with the result being the
ability to rapidly rev up to speed,
and just as quickly come to a stop.
Another nice aspect of the coreless
motor is the absence of the annoying
cogging as the rotor poles pass by the
field magnet poles, allowing smooth
slow speeds. Though brushed, the
lack of the iron core dramatically
reduces rotor inductance and
commutator arcing. This, in turn,
reduces EMI noise in the control
circuitry. Coreless motors are usually
small and are popular in the better
lines of hobby servos. Figure 9 from
GlobalSpec shows the unique basket
weave winding of the rotor.

FIGURE 6. C40-300 Magmotor.

experimental robot construction. The
selection of the proper motor for your
particular design or application should
be preceded by a thorough study of
motor manufacturer’s websites and
some of the more popular robot
reference books such as Gordon
McComb’s Robot Builder’s Sourcebook.
Motors can range from the tiny vibrator
motors used in cell phones and pagers,
to the huge Magmotors. All have their
place in the design of experimenter’s
robots and are vital components that
give our creations motion. SV

Winding Down

Tom Carroll can be reached via email
at TWCarroll@aol.com

These are just a few of the
most popular motors used in

FIGURE 8. Brushless motor.

FIGURE 9. Coreless DC motor.

Coreless DC Motors

Coreless motors have a great
advantage of low inertia in the moving
rotor and a low mechanical time